Electromagnetic (EM) transmission frequency bands are allocated so that systems operating on different bands do not interfere with each other. In practice, however, interference may occur between transceivers operating in different bands because, inter alia, a transceiver nominally operating within one band generates EM frequencies outside the band. Furthermore, if the transceivers are physically close to each other, interference may occur regardless of the frequencies at which the transceivers operate.
FIG. 1 is a schematic diagram of the frequency spectrum of two frequency bands which are relatively close to each other, as is known in the art. A first band 12 is an upper Industrial, Scientific and Medical (ISM) band, operating at frequencies of approximately 2.4 GHz. A second band 14 is a Multichannel Multipoint Distribution Service (MMDS) band, operating in an approximate range 2.596-2.680 GHz. Both bands are used for implementation of broadband wireless access systems, which are intended to provide digital services such as Internet access, wide-area networks (WANs), and Voice-over Internet Protocol (VoIP).
Chapter 15 of a protocol ANSI/IEEE 802.11 (1999) published by The Institute of Electrical and Electronics Engineers, Inc. New York, N.Y., specifies 2.400-2.497 GHz as frequencies within the ISM band to be used in wireless local area networks (WLANs). The specified frequencies are shown schematically as region 16. The Institute of Electrical and Electronics Engineers, Inc. publish supplements 802.11a, 802.11b, . . . to the ANSI/IEEE 802.11 protocol. Hereinafter, the term ANSI/IEEE 802.11 protocol is assumed to comprise the supplements.
Chapters 9 and 11 of the ANSI/IEEE 802.11 protocol, describe functions of a medium access control (MAC) sub-layer of stations operating according to the protocol. Operations of the stations are timed, and stations maintain local clocks which are periodically synchronized by a “beacon” frame transmitted by an “Access Point” (AP) station, acting as a timing master for all the stations. The beacon frame is typically followed by one or more management frames which are also referred to hereinbelow. As described in chapter 9, the protocol defines a contention period and a contention-free period for operation of the stations. The contention period is implemented by a distributed coordination function (DCF) in each station. The DCF operates a carrier sense multiple access with collision avoidance (CSMA/CA) system, wherein a station wanting to transmit first “senses” the medium to determine if another station is transmitting. If the medium is determined to be available, the transmission may proceed.
The contention-free period is implemented by a point coordination function (PCF), which is optionally implemented in the AP station. When implemented, a PCF AP station transmits a management frame which gives control of the medium to the AP station, and enables the AP station to poll the other stations. The management frame prevents non-PCF stations from transmitting by setting a network allocation vector (NAV) of each station. The NAV is an indicator, maintained by each station, of time periods when transmission will not be initiated by the station.
A protocol TIA/EIA/IS-856, published by the Technical Specification Group C of the Third Generation Partnership Project 2, and which is available from the Telecommunications Industry Association, Arlington, Va., gives specifications for code division multiple access (CDMA) transceivers supporting the protocol. Although not in the specification, it is known in the art for CDMA transceivers to operate in two sections of the MMDS band in the 2.6-2.7 GHz range. In each section, shown as regions 18 and 20, the CDMA transceivers may transmit and receive.
Chapter 9 of the protocol, which is incorporated herein by reference, describes a data rate control (DRC) channel that indicates a rate at which an access terminal can receive traffic in a specific channel. The rate may be set to any of a pre-determined set of values including zero. Setting the rate to zero effectively prevents the channel which is “pointed to” by the DRC channel from receiving.
Particularly in cases where the transceivers are relatively close physically, a WLAN transceiver operating in region 16 may generate interference in a CDMA transceiver operating in region 18, and vice versa. There are four main reasons for the interference:                Low receiver selectivity, causing the receiver to be unable to distinguish signals directed to the receiver in the presence of signals of different frequencies.        Insufficient receiver blocking handling, where receiver operation is degraded due to strong signals, different from the tuned frequency of the receiver, input to the receiver.        Transmitter out-of-band emissions, where significant power is emitted from the transmitter due to insufficient filtering.        Transmitter wide band noise.        
Methods for reducing interference between a WLAN transceiver and a CDMA transceiver which are physically close typically comprise using high quality filters (radio-frequency (RF), intermediate frequency (IF) and baseband) and careful RF design. Both methods lead to increased transceiver costs. Furthermore, RF solutions are not able to solve the problems generated by the close physical proximity of transceivers. Thus, an alternative system for reducing interference which avoids these costs and which overcomes the problems caused by the proximity of the transceivers would be advantageous.